The present invention relates to a method and mechanism for producing suction and periodic excitation flow and, more particularly, to causing periodic oscillation of an amplified flow emanating from a jet port between two or more defined exit directions.
Flow control technology relates generally to the capability to alter flow properties relative to their natural tendency(ies) by introduction of a constant, or periodic, excitation. Use of a periodic excitation for control of boundary layer separation has been demonstrated to be both possible and efficient in incompressible flows (1, 2) especially at low speeds and in a wide range of Reynolds numbers (Re; 104–107).
Control of boundary layer separation in compressible flows has also been demonstrated, although the level of oscillation required is higher than that required in in-compressible flows (3, 4). Despite this, as long as the flow is free of shock waves, there is no theoretical or physical difference resulting from the mere increase of Mach number. One of the primary uses of flow control is boundary layer control to prevent unwanted boundary layer separation.
Significant scientific and technological effort has been invested in control of boundary layer separation. Alternate methods of flow actuation have been examined including mechanical mixing (e.g. vortex generators, Allan et al (2002) Numerical Simulations of Vortex Generator Vanes and Jets on a Flat Plate, AIAA Paper 2002–3160), pneumatic vortex generatorjets (e.g., steady and oscillatory, Johnston, et al. (2002) International J. of Heat and Fluid Flow, 23(6):750–757 ; and Khan and Johnston, (2000) International J. of Heat and Fluid Flow, (21(5): 505–511.), and cyclic excitation. In an external flow, and at low Re. it has been demonstrated that cyclic excitation is more efficient than steady excitation for boundary layer control by about two orders of magnitude (1). FIG. 1 (1) shows the influence of steady wall jet (solid line) and periodic excitation (dashed line) on the lift generated by a wing profile beyond the stall angle.
In cases where the boundary layer control of a compressible flow is required, there is an urgent need for periodic excitation actuators (PEC) with strong output and suitable frequency range. It is expected that there will be a requirement for excitations with strength comparable to the speed of the flow at the boundary layer edge near the separation region, and frequencies in the range of 100 to 2000 cycles per second. Although valves that operate in this frequency range are available, these valves fail to produce the required excitation strength at the appropriate frequency range. Further, such valves are inefficient and difficult to incorporate into modem jet propulsion systems.
Unsteady flow control of a compressible flow requires an excitation strength that approaches the speed of the flow to be controlled, and a frequency that creates a Strouhal number on the order of 1 (lower limit of 0.25 and upper limit of 0.55), based upon the length of the separated region and the free-stream velocity. Assuming that a flow with a speed 0.7 Mach number is to be controlled, and the length of the separated region is c=0.2 meter the required frequency,ƒ, is described by Equation 1 (for standard atmosphere sea-level conditions).f=StU∞/c=1*340*0.7/0.2=1190 Hz.  Equation 1.
Creation of excitations with a frequency in this range is possible with Piezo electric flow generators (6) and by mechanical chopper devices (2). However, the maximum intensity of the flow generated by these methods is in the range of 0.3 Mach number. This means that these methods are ill suited for use in control of boundary layer separation in compressible flows, supplying only about half of the required flow output strength, or less than a quarter of the required oscillatory momentum (4).
Mechanical excitation generators that interact directly with the boundary layer (7) have also been tested in this context. However, these devices have, as an inherent disadvantage, a dependency on the velocity gradient of the boundary layer (or more generally the shear-flow) at low speeds and their output periodic excitation capability is limited and for most applications insufficient.
Two additional types of flow actuators (8, 9) relying upon trans- and supersonic flow output speeds are being developed and should be capable of providing the required flow intensity, and more. These supersonic actuators rely upon release of a large quantity of energy in a short time into a small internal cavity inside a body connected to the exterior flow by means of a hole(s) or a slot(s). The first type of actuator relies upon cyclic explosion of flammable materials (as in internal combustion engine) and the second type of actuator relies upon creation of an electrical discharge (as in spark or ark generators) of great magnitude in a small space during a short time and with a defmed repetition rate. The first actuator is currently limited by an upper frequency of 100 cycles/second (e.g. U.S. Pat. No. 6,554,607 to Glezer et al.).
The second type of actuator is similar to the first type, but the entire energy deposition is due to an electric discharge. It remains untested with respect to its ability to cyclically generate the required output flow. These two actuator types share, as inherent disadvantages, a strict requirement for rare materials which are suitable for high temperatures and an undesirable thermal (and perhaps radiant) influence on the surrounding environment. In addition, the requirements for auxiliary cooling systems and the electromagnetic influence on other systems have not yet been determined.
Pneumatic valves that employ compressed air have been developed and demonstrated to be suitable for flow control (10). These pneumatic valves have been applied to compressible flows and it has been concluded that their low energy efficiency will prevent any effective development for use in boundary layer control because of the great pressure differential required by the valve in order to generate the oscillations. This great pressure differential (or loss), even if it can be achieved, would require the use of a rigid durable structure which would be too heavy for use in many applications (e.g. aviation). The combination of pressure differential and increased weight reduce the efficacy of this approach so that any potential advantage to be realized form prevention of boundary layer separation is obliterated.
Flow control dates back to the discovery of the boundary layer by Prandtl. In his historic lecture of 1904, he defmed the boundary layer and the scientific and engineering advantages to be realized from this revolutionary new idea. Prandtl also defined the basic theoretical problems related to control of boundary layer separation. Prandtl went on to explain the solution to these problems, control of the boundary layer separation by suction, applied upstream of the separation point with suppression of the negative phenomena resulting from the flow detachment from the surface. These phenomena leads to reduction in efficiency of the flow related mechanism. Prandtl demonstrated the efficacy of the concept of suction of the boundary layer by placement of suction slots upstream to the boundary layer separation point in a wide angle diffuser, whose boundary layers separated without suction. In the presence of Suction, the flow remained attached to the two walls of the diffuser (5).
Even in a case where suction of the boundary layer prevents separation locally, the adverse pressure gradient becomes larger in many cases and increases geometrically, requiring significant spreading of the flow streamlines and causing boundary layer separation downstream of the point where suction is applied.
The aerodynamic efficiency of suction of the boundary layer has been proven (11) but remains problematic from the standpoint of maintenance in cases where a suction pump is required. Part of the suggested solution from the second half of the 20th century is to combine suction from one place with exhaust in another place, in the case of boundary layer control by a steady wall jet.
More recently, (1) it has been proven that boundary layer control by means of cyclic excitation, without mass additions (i.e., zero-mass-flux) is more efficient by two orders of magnitude than the efficiency of boundary layer control by means of a steady wall jet that does not oscillate (FIG. 1).
In contrast, it has been proven that the combination of suction and periodic excitation (with a small but negative averaged mass transfer) increases the efficiency of periodic excitation that serves to control the boundary layer (12), as compare to zero-mass-flux excitation.
In additional experiments (1, 13) it was proven that addition of momentum that “rides” on the excitation frequency does not reduce the efficiency of the periodic excitation with respect to boundary layer control with zero-mass-flux as long a Cμ>0.2% (see Equation 2).
                                              ⁢                              C            μ                    =                                                    A                ex                                            A                wing                                      ⁢                                          (                                                      U                    p                                                        U                    ∞                                                  )                            2                                                          Equation        ⁢                                  ⁢        2            Where:                Aex is the exit cross section area of the excitation device(s);        Awing is the reference area of the controlled flow;        Up is the slot exit peak excitation velocity; and        U∞ is the free-stream velocity.        
Thus, according to what is currently known, it would seem that in order to control the boundary layer in a compressible flow with speeds in the range of Mach numbers between 0.3 and 0.7, the best combination would be suction near the boundary layer separation point and cyclic exhaust of the same (or amplified) fluid downstream of the suction slot. Implementation of the recommendation will lead to control of the boundary layer in a compressible flow, in a flow that requires a high level of control (e.g. excitation Mach numbers). All of the above considerations apply also to incompressible flow where significant control authority is required.
Because this recommendation employs a valve with a negligible pressure differential and an unimpeded flow path, with no significant turns, it does not seem that there is a limit to the flow speed at the exit from the valve as long as the flow is free of shock waves. To date, exit speeds in excess of 200 m/s have been measured.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and mechanism capable of overcoming the above limitations.